While I don't have children just yet, Sebastian is the same age I was when my parents had my little sister, and I acted very similar to how he did for a time.

I loved being a big sister, but I know that I used to find it unfair that Mikayla seemed to be the centre of attention with our other family members - I was always Nana's little girl, and I would always do special things with my dad's mum as well, and even though I had other cousins, I was the eldest on my dad's side, and the closest/one who visited the most on my mum's. When my sister was born, that changed for me, and I used to act out, because, at that age, most kids don't really know how to explain how they feel or convey their emotions as well - frustration isn't easily explained by a 4 year old. It was worst with babysitters, when I was about 7 and Mikayla was 4, we had one who only cared about my sister, and refused to acknowledge my presence, until I started doing really drastic things that were out of character for attention (such as hopping up on the counter and threatening to jump off it).

He is likely frustrated and confused by all the changes, while you and your husband do still give him all the cuddles he had before, he will likely need some more time adjusting, 3 new babies is enough to confuse any little one! It took me about a few months to adjust to my little sister - so I wouldn't be too concerned that he hasn't adjusted yet. Acting out is just his way of letting out frustration and confusion by the new lifestyle. Is he involved in a play group? Or, maybe a 'Mummy and me" or "Daddy and me" day will help - my parents used to try and take me out for one, or play one of my favourite games with me for some one on one time, and it's also a good opportunity for him to ask questions about the babies that he might not have been sure on how to ask, or for you to ask him questions ("are you excited to have the babies?" "would you like to help me give them cuddles when we get home?", etc). If you try to slowly get him used to the babies, he'll probably adjust on his own time. Forcing him into it is the last thing you'd want to do, so if he shows an interest, help to sort of, guide him into spending time with the babies.

My husband and I have made certain that the boys' nighttime routines haven't changed, despite moving Sebastian into our bedroom. Kaidan has adjusted just fine to having his own room, though he's asked a couple of times why his big brother gets to sleep in our room. There's been nights that they've both been in with us, each with a bed on the floor. Sebastian liked it at first, and it was really helping him to have "sleep-overs" with Mommy and Daddy. Now he just gets mad when we get up to take care of the babies.

The babies' nursery is directly off of our bedroom, and I think that has started bothering Seb because he says we need to move them. Him and Kaidan pretty much treat each other as equals, though they both have traits of the oldest sibling, because biologically they are. Most of the time Kaidan just goes along with whatever Seb wants.

We tried giving him control of little things, like helping pick outfits or helping with lunch and dinner time routine. He's very much a routine lover, just like me. Since bringing the babies home things have changed, but it hasn't been to drastic. He gets his quiet time with just Daddy when my husband gets home from work, and story time with us and brother before bed. Morning activities are the same, crafts and games. Sometimes led by Hannah though. It's been wonderful to have her. Currently the babies nap right through dinner, so the tradition we have of a family for sitting down together at meal times is also the same.

My mother in law offered to let him start sleep overs, but I'm not sure I'm ready for that! Do you think he's to little for that?

When we just had Evelyn at home he loved being a big brother, and wanted to help with absolutely everything. Kaidan could really care less about the babies, he loves on them but would much rather focus on his toys.

I don't know how much more control we can give him, without him becoming superior to my husband and I. Boy is he stubborn! I might ask my husband what he thinks about having a "Daddy and Me" kind of day on Saturday, give Sebastian some solid uninterrupted attention for a few hours from his Daddy. We've actually enrolled both the boys in three day a week preschool, which doesn't start until September. We're hoping that this time away with help Seb when he is at home with his siblings.

My son was younger when he became a big brother but he had a rough time adjusting too. We tried a lot of things and none of them really worked magic or anything but as time passed things got easier. One thing we did was try to ignore minor forms of acting out. I didn't want him to be constantly in trouble or to feel like all we ever did was correct him so we let some little things slide. We also started doing a behavior chart where he got stars for certain behaviors like no whining, setting the table, not harming others and getting dressed by himself. Our biggest issue with him was biting the baby so he got a stars on days he didn't hurt her. In the end we actually took him to a behavioral therapist, mainly because the biting. It was expensive but it worked. We started doing special time where we played with him for five minutes each day and praised him ten times in those five minutes, narrated what he was doing ten times and reflected things he said ten times. After a couple months of doing this we added five minutes of listening time where we gave commands that he had to follow or else he was sent to time out. It gave him a chance to practice listening. Once he was able to do it during listening time then we stopped listening time and got stricter about his behavior throughout the day. We also did a behavioral chart where he got to move Thomas every couple hours across a track if he didn't hurt anyone. This worked wonders with his behavior and he loved moving Thomas. When Thomas reached the station, which could happen up to two times a day he got a coin for his piggy bank.
You mention your MIL taking him. He's older then my son was when my daughter was born so it might be different but my in-laws watched Patrick when Mary was six weeks old overnight because I had my six week appointment early the next morning and it just made it easier not to have to drive out to their house and drop him off then head in the opposite direction for my appointment all with rush hour traffic. He didn't do very well with staying there overnight. He had like five accidents the next day and his behavior was very challenging. Even more so then normal. Of course all kids are different so your son might not react so strongly to going somewhere else overnight.

Just a note on the overnights... You may wanna try it before school starts, just to see how it goes. My sons spends occasional overnights at my MIL's home, and he does super. He is extra tired the next day, but he sleeps well there, is happy, and behaves himself. Travis turned 2 the beginning of May.
However, you won't wanna do anything to feed Seb's insecurity. It's totally a decision for you and your husband. You two know Seb the best, how comfortable he is at his Grandmother's home, with her in general, and his overall maturity level.
You could also ask your MIL to do occasional trips to the park, exclusive playtimes at her home, etc, if you feel he is not ready for overnights just yet. Sometimes Kaiden could go along, and other times not.
The idea of the reward chart is interesting, too. Seb's old enough that might work.
Great idea to have your hubby spend a day w/ him! I just want to reiterate you are doing a good job. I'm sure things will improve for everyone with time. And preschool will surely help. God bless you!